Although a lithium ion secondary battery is an excellent battery having a very high energy density, it has danger of explosion when a short is generated, such that a separator used therein is greatly requested to have quality stability together with a high quality level. In accordance with the recent trend of high-capacity and high-power of the lithium ion secondary battery such as a battery for a hybrid vehicle, etc., there has been a greater demand for a thermal stability of the separator, in addition to the quality stability of the conventional product. This is the reason that if thermal stability of the separator is deteriorated, the danger of explosion by the melt-down of the separator due to overheat of the battery is increased.
The efforts to improve thermal stability of the separator have been tried broadly in three ways. There are a method to increase thermal properties by adding inorganic materials or resin having thermal resistance to the conventional polyethylene, a method to coat material having thermal resistance on a surface, and a method to prepare a multi-layer separator including a layer having thermal resistance.
U.S. Pat. No. 6,949,315 discloses a film that improves thermal stability of a separator by mixing 5 to 15 wt % inorganic materials such as titanium oxide to ultra-high molecular weight polyethylene. However, the method suggested in the above-mentioned patent has an effect in slightly improving thermal stability by adding the inorganic materials but thermal stability is not improved to the level higher than a melting point of polyethylene since the matrix of the separator is polyethylene. Also, this method may easily cause problems such as deterioration of mixing performance due to adding of the inorganic materials, generation of pin-holes, and non-uniform of the quality due to the deteriorated mixing performance when being stretched. Also, the method may cause deterioration in the physical property of the film such as impact strength due to lack of compatibility of interface between the inorganic materials and polymer resin.
A separator that is prepared by mixing a resin having excellent thermal resistance instead of inorganic materials is disclosed in U.S. Pat. No. 5,641,565. This technique is to mix 30 to 75 wt % organic liquid and 10 to 50 wt % inorganic materials with a resin mixture where polyethylene is mixed with 5 to 45 wt % polypropylene and then to extract the organic liquid and the inorganic materials, thereby preparing a separator. This technique has a limitation in improving thermal stability since the matrix of the separator is polyethylene. Also, the physical property is deteriorated due to the addition of polypropylene that does not mix with polyethylene as mentioned in the patent itself. This method needs a relatively high amount of polypropylene in order to obtain a sufficient thermal stability, wherein the physical property of the separator is more deteriorated.
A method to coat material having thermal stability on a surface of a microporous film is disclosed in U.S. Patent Publication No. 2006-0055075. However, since the coating method has a limit in increasing the permeability of a coating layer, the permeability of the entire film is highly likely to be decreased and the non-uniform quality is highly likely to be generated due to the deterioration in a wetting property between the coating layer and the microporous film.
The method to prepare a multi-layer separator for improving thermal stability of a separator is to use lamination. U.S. Pat. No. 5,691,077 discloses a method of making a 3 layer separator by laminating a polypropylene layer having a high melt-down temperature (having a high melting temperature) on polyethylene having an excellent shut-down property (having a low melting temperature). Although this separator prepared by a dry process (a method for forming micro pores by causing micro crack between lamellas as a crystalline part of polyolefin through stretching at low temperature after preparing a thick polyolefin sheet) is excellent in view of thermal properties, it has not only a deterioration problem in productivity due to the addition of the lamination process performed in a separated process but also a delamination problem due to the defect of the lamination, in addition to the disadvantages of the stretching non-uniformity, the generation of pin-holes, and the increase of thickness deviation, etc. in a preparation process of a fabric film by a dry process, such that this separator has not been widely used. Also, since the separator prepared by the dry process has a small pore size, it is difficult to provide sufficient permeability.
Japanese Patent Laid-Open No. 2002-321323 discloses a polyolefin microporous film that polyethylene layer and polyethylene/polypropylene mixed layer are laminated and unified. However, the polypropylene content of the layer made from polyethylene/polypropylene mixture is so low, that it is difficult to expect sufficient increase of the melt-down temperature.
A separator for a secondary battery essentially requires an electrolyte retaining property and a battery assembling stability in addition to strength, permeability, quality uniformity, and thermal stability. However, the above-mentioned prior arts do not have the properties at the same time.